1. Field of the Invention
The present invention relates to fastening of an inner blade section and outer blade section to a pitch system of a partial pitch wind turbine blade, in particular to the bolting connection.
2. Description of Related Art
In certain wind turbine blade designs, it is desired to be able to pitch some or all of the wind turbine blade body for controlling wind turbine operation. Such pitching is accomplished through use of a pitch system provided between the turbine sections desired to pitch.
With reference to FIG. 1, a cross-sectional illustrative example of a pitch system coupling installed in a wind turbine blade is indicated by 50. Only one side of the pitch system 50 is shown in cross-section in FIG. 1—it will be understood that the entire pitch system 50 continues in a complete circle, constituting an entire pitchable coupling between adjacent blade sections.
The pitch system 50 comprises first and second concentric pitch rings 52, 54 coupled to each other via at least one bearing runway 56. The first pitch ring 52 is an outer pitch ring having a relatively larger diameter than the second pitch ring 54, which is located within the circumference of the first pitch ring 52. The bearings in the runway 56 permit the first pitch ring 52 of the pitch system to rotate relative to the second pitch ring 54.
In FIG. 1, the first pitch ring 52 is coupled to a first blade section 58; a bolt 60 is extending through the first pitch ring 52 and securely received within the end portion of the first blade section 58; a nut 62 is applied to the free end of the bolt 60 for tightening against the first pitch ring 52 and the underlying end surface on the first blade section 58. Similarly, the second pitch ring 54 is coupled to a second blade section 64; a bolt 66 is extending through the second pitch ring 54 and securely received within the end portion of the second blade section 64; a nut 68 is applied to the free end of the bolt 66 for tightening against the second pitch ring 54 and the underlying end surface on the second blade section 64. Consequently, the second blade section 64 may be rotatably pitched relative to the first blade section 58.
As only one side of the pitch system 50 is shown in cross-section in FIG. 1, accordingly, it will be understood that a plurality of bolts 60, 66 and nuts 62, 68 are provided about the circumference of the pitch system 50.
As a result of the described configuration of pitch system 50, one set of a plurality of nuts and bolts are located on the exterior of the blade—in this case the first set of nuts and bolts 60, 62. This configuration is the standard arrangement for full-pitch wind turbine rotor blades (where the pitch ring is provided at the root end of the rotor blade), for example as seen in U.S. Pat. No. 6,783,326. Similarly, the arrangement of one set of nuts and bolts on the exterior of the wind turbine blade is also used for partial pitch rotor blades, where the pitch system is located at a point on the longitude of the entire rotor blade between a first blade section and a second blade section.
One problem with such an arrangement, however, is that any maintenance of the bolting connection of the first pitch ring 52 requires external blade access to correctly position a worker to perform suitable maintenance operations, e.g. tightening of the bolt connections. Such external blade access results in considerable maintenance time and cost, as blade rotation must be securely and safely stopped. Furthermore, in the case of partial pitch turbines a gantry or crane-hoisted platform must be positioned adjacent the pitch junction of the blade under maintenance. In the case of off-shore turbines, such maintenance operations involve additional costs, as any external maintenance operations are more susceptible to weather delays, etc. As a maintenance operation, such as bolt tightening, must be performed regularly during the lifetime of a wind turbine, the costs involved in such maintenance operations represent a considerable operational expense for such wind turbines.
In International patent application WO 2005/019642, a wind turbine blade pitch system is disclosed and illustrated by a blade mounted rotatably to the hub. The connection between the blade and the hub is made through a pitch drive disc. This disc is mounted to the blade by bolts that extend into the blade. Between the hub and the disc, there are provided other bolt systems that extend through the disc and through a wall of the hub. The disclosure also contains an illustration of a pitch system between two parts of a blade. No information is given, how these bolts are fastened. Also, International patent application WO 2011/056121 discloses a wind turbine with a pitch system between two parts of a blade.
It is an object of the invention to improve the prior art systems. Especially, it is an objective to provide a wind turbine blade which overcomes the problems outlines above and results in a reduction in associated maintenance costs.